Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of securities sold under agreements to repurchase
Amount outstanding at December 31	$ 191,390	$ 143,320	$ 108,550
Weighted average rate at December 31	0.17%	0.24%	0.36%
Maximum amount outstanding at any month end	213,730	152,267	239,830
Daily average balance outstanding during the year	$ 174,624	$ 129,137	$ 133,080
Weighted average rate during the year	0.21%	0.29%	0.43%